The invention relates to a method for setting the suction pressure in a refrigerant circuit of an automobile, in which a current suction pressure is determined and is compared with a suction pressure to be set, the delivery rate of a refrigerant in the refrigerant circuit being set as a function of the comparison, and to a device for carrying out the method.
DE 38 43 924 C2 discloses a generic air conditioning system for an automobile. This air conditioning system has an engine and a compressor of variable capacity. The compressor is a swashplate compressor, in which the angle of inclination of the swashplate is modified and the displacement of the compressor is thus varied. With the aid of a detector device, the thermal load on the system is recorded and the necessary suction pressure of the compressor is determined as a function of the thermal load recorded. The desired suction pressure is corrected by varying the displacement of the compressor.
When the automobile engine is in the idling state, the idling speed of the engine is set according to the delivery rate of the compressor, in order to prevent the engine from stalling.
However, the disadvantage of using displacement-controlled compressors is that the influence of the back expansion volume on the control circuit is not taken into account.
In all controlled reciprocating piston compressions, the design-induced back expansion volume (switching space, dead volume) is of great importance, since, with increasing cutoff (that is to say, dead center shift), the ratio of back expansion volume to current displacement becomes more and more unfavorable. As a result of this, during cutoff, the volumetric efficiency, and therefore also the refrigerating capacity/compressor drive power ratio, becomes increasingly lower, thus leading to high energy consumption.
The object on which the invention is based is, therefore, to specify an air conditioning system, the energy consumption of which can be set optimally and which can nevertheless be produced by means of simple design.
The object is achieved, according to the invention, in that the suction pressure to be set is varied by regulating the speed of a drive element, the suction pressure to be set being determined as a function of the relative air humidity.
Advantageously the suction pressure can be modified continuously.
The advantage of the invention is that, due to the variable change in the desired value of the suction pressure, the same level of comfort is achieved by expending less energy.
In a further embodiment of the invention, a device for operating a refrigerant system in an air conditioning unit for an automobile has an evaporator, through which the air to be cooled flows, the air being cooled via a refrigerant, the delivery rate of which is set by means of a compressor operated by a drive. According to the invention, the speed of the drive, operated independently of the automobile engine, is determined as a function of a suction pressure to be set on the compressor.
The invention has the advantage that the compressor is operated in the part load range under conditions which are beneficial in energy terms, thereby achieving a saving of energy in the automobile. There is no need for the air temperature downstream of the evaporator to be reheated in order to achieve a comfortable interior temperature. At the same time, the ratio of refrigerating capacity to compressor drive power is set optimally. Control units arranged additionally on the compressor for setting the suction pressure are not necessary.
In a particular embodiment, a regulating device, which compares the current suction pressure with a suction pressure to be set, varies the speed of the drive during the entire operating period of the compressor as a function of the comparison.
Advantageously, the compressor and the speed-regulated drive, which, for example, is an electric motor, form a structural unit, which is accommodated in the automobile independently of the engine. It is thus possible for an air conditioning system to be retrofitted with such a speed-regulated compressor in a simple way.
Such structural units have low weight and small geometric dimensions. By means of this structural unit, a hermetically closed-off refrigerant system can be produced, this being advantageous, in particular, from the point of view of environmental protection.
In one embodiment, the regulating device activates the electric motor in a clocked manner. Activation which can be implemented in a simple way may be achieved, in particular, if the clocked activation of the electric motor is carried out by varying the pulse width of the activating signal generated by the regulating device.
In one development, the speed-regulated compressor has a fixed displacement. Compressors of complicated design, for example with a swash plate, may thus be dispensed with. The suction pressure is determined solely by the speed.